The present invention relates to an optical mask for manufacturing a semiconductor, and more specifically, to a phase shift mask including a phase shifter formed on a transparent film in a predetermined pattern and a method for correcting defects thereof.
In a conventional photomask, a light intercepting pattern 2 of chromium or the like is formed on a transparent substrate 1 such as quartz. However, as a semiconductor pattern has been miniaturized, the improvement of a pattern resolution in a lithography process has been demanded, and thus a phase shift mask having such a basic structure as shown in FIG. 1 has been put into practical use, wherein a transparent pattern 4 for providing a phase difference to an exposure light is formed at specified gaps between two adjacent light intercepting pattern portions.
When a mask has defects, the defects are transferred to all of the semiconductor chips exposed with the mask, and accordingly the mask defects must be perfectly removed. The defects of a phase shift mask to be removed include not only the conventional omission of light intercepting portions and occurrence of excessive light intercepting portions, but also the omission of shifter portions and occurrence of unnecessary shifter portions. Among these defects, those other than the omission of the shifter portions can be corrected by local sputter etching using a focused ion beam and by film formation using a local chemical vapor reaction. However, for correction of the omission of the shifter portions, it is necessary to employ a method of providing a sub-shifter under a usual shifter or the like, as described in the article "High Resolution Light Lithography Technology Using Phase Shift Method" Journal of the Institute of Applied Physics, Vol. 60, No. 11 (1991), pgs. 1076-1086. Further, it is anticipated that a method of using a focused ion beam will be successful for the correction of fine local portions. This method has been applied to correct the defects of a conventional optical mask. Japanese Patent Application Laid-open No. 58-56332 describes a basic concept regarding the correction of the defects of the optical mask. Further, Japanese Patent Application Laid-open No. 3-196041 describes a defect correcting method for an optical mask wherein defect portions are etched to a certain depth such that phases of light passing through both the etched portion and the normal phase shift portion are the same so that a non-defect optical mask can be obtained.
The correction method reported by the aforementioned article is carried out by the following procedure. The structure of a phase shift mask is such that a silicon nitride film having a film thickness capable of inverting the phase of an exposure light by 180.degree. is formed on a quartz substrate 1 as a sub-shifter 12 as shown in FIG. 2(a). A light intercepting pattern 2 of chromium is formed on the silicon nitride film 12. Then, a shifter pattern 4 of silicon oxide for inverting a phase by 180.degree. is formed at specified gaps of the light intercepting pattern 2. The shifter 4 has defects such as a shifter omission portion 5 and a shifter remaining portion 6 formed in the process of forming the shifter 4. In the method for correcting these defects, first, a resist 13 is coated on the whole surface of the mask. The defective portions 5, 6 are then subjected to spot-exposure, and developed to form a resist pattern as shown in FIG. 2(b). This resist pattern is subjected to an anisotropic dry etching to remove the shifter 4 of silicon oxide, as shown in FIG. 2(c). In this dry etching, an etching gas capable of obtaining selective etching between silicon oxide and silicon nitride is used for stopping the etching on the surface of the sub-shifter 12. Conversely, using an etching gas having such a selection ratio that the processing speed of silicon nitride is greater than that of silicon oxide, an anisotropic dry etching is performed at the shifter omission portion 5, thereby removing the sub-shifter 12 formed on the underlayer by the silicon nitride. At this time, the etching stops on the upper surface of the substrate 1 because the substrate 1 under the silicon nitride is composed of quartz, i.e., silicon oxide. After the completion of the above processes, the resist 13 is removed, to thus obtain the corrected state as shown in FIG. 2(d). Before the correction of the defects, the phase of the exposure light transmitting through the shifter 4 becomes an intermediate value between 0.degree. and 180.degree. at the defective portions 5 and 6, and consequently, shading is formed upon exposure. However, by the removal of the shifter remaining portion 6 from the sub-shifter 12, the shading due to this portion 6 disappears. Further, with respect to the shifter omission portion 5 caused at the portion where the shifter 4 is overlapped on the sub-shifter 12, it is removed up to the sub-shifter 12, and thereby the phase difference becomes 0.degree.. On the other hand, at the portion where the shifter 4 is overlapped on the sub-shifter 12, a phase difference is 360.degree.. Accordingly, the phase difference at these regions is substantially removed, and thus shading due to the shifter omission portion 5 disappears. As described above, the shifter 4 can be corrected by this method. Nevertheless, this method has such a problem as being complex in its correction process, thereby involving a fear of causing new defects, and it is difficult to accurately perform the positioning of the exposure by spot-exposure in such a complex pattern.
A process using a focused ion beam (hereinafter referred to as FIB) is suitably applied to simplify a correction process and to improve the accuracy of correction. FIGS. 3(a)-3(c) show processes for correcting defects by using an FIB 7. In these processes, the shifter remaining portion 6 is removed by sputter etching by the FIB 7. Further, the shifter omission portion .5 is also removed by sputter etching by the FIB 7; however, in this case, the phase shift of this portion is 0.degree. in this state, and thus there occurs phase-inversion between the phase shift 180.degree. of a shifter portion 4 not processed and the same, thereby producing shading upon exposure.
To cope with this problem, as described, for example, in Japanese Patent Application Laid-open No. 4-449, there is adopted the following correcting method: namely, at the shifter portion 4 of the shifter omission portion 5 removed by the FIB 7, the substrate 1 is deepened by an amount providing a phase shift of 180.degree. to substantially remove a phase difference between the shifter portion 4 and the region where the shifter omission portion 5 is corrected, thus preventing shading from being. Nevertheless, with this method, when the shifter omission portions 5 having indefinite configurations, that is, being different in height from each other are corrected, the substrate 1 is processed according to the irregularity depending on the configuration of the shifter 4 as it is, and thus phase disturbances are sometimes generated at the corrected portions, which makes it difficult to perform perfect correction.
Next, as a method of solving the problem in the processing using the FIB, there is adopted a method of using an FIB assist etching (hereinafter referred to as FIBAE) as described in the copending application and as shown in FIGS. 4(a)-4(c). According to this method, a stopper layer 3 is inserted between the shifter 4 and the chromium pattern 2 to absorb the irregularity of the shifter 4 when using the FIBAE. For example, when a substrate 1 is composed of quartz, a shifter layer 4 is composed of a spin-on-glass (SOG), a gas nozzle 8 supplies a reactive gas 9 used for the FIBAE such as XeF.sub.2 (xenon difluoride), and the stopper layer 3 is composed of Al.sub.2 O.sub.3, MgF.sub.2, CeF.sub.3 or the like, etching is effected slowly to the shifter layer 4, and thus the processing substantially stops on the stopper layer 3 as shown in FIG. 4(b). Thus, at this step, the irregularity of the shifter 4 is changed to the smaller irregularity in the stopper layer 3 which is thinner than the shifter 4. Thereafter, as, for example, disclosed in Japanese Patent Application Laid-open No. 4-449, the application of the reactive gas 9 is stopped, and the processing is changed to the FIB having little selective etching as shown in FIG. 4(c). Thus the substrate 1 is deepened for eliminating the phase difference between the portion where the shifter omission portion 5 is corrected and the portion having the shifter 4, and therefore, shading produced by the shift omission portion 5 is removed upon exposure. According to this method, since a bottom surface can be flatly processed, the defect correction can be performed with high accuracy in a depth direction regardless of the configurations of defective shifter portions.
Further, by use of the FIB 7 capable of focusing an ion beam to 0.1 micron or less, the accuracy of correction on a flat surface can satisfy the requirement for defect correction. Nevertheless, a problem of this method lies in a resistance to cleaning of the stopper layer 3. In general, as shown in FIG. 5, in manufacturing a phase shift mask, a patterning process is invariably followed by a washing or cleaning process. In the cleaning process, a strong cleaning such as an ozone sulfuric acid cleaning is carried out to perfectly remove the contamination of a mask. For example, Al.sub.2 O.sub.3, a general coating material, is not resistant to this cleaning, and thus Al.sub.2 O.sub.3 which is not under a chromium pattern 2 is perfectly dissolved in the cleaning process after the formation of the chromium pattern 2. Therefore, the stopper layer 3 does not exist under the transmitting portion of the shifter 4 to be corrected, and thus the correction to which the above FIBAE process is applied becomes impossible. However, this method can be employed when the stopper layer 3 is selected from materials having such characteristics as transmitting an exposure light, having a large selectivity in the FIBAE process, being resistant to the cleaning, and forming a practically usable film.